The corneal is a transparent dioptric media without vascular tissue. It is the most effective refractive surface of the eye optical system and accounts for approximately 70% of the eye's total refractive power. The refractive power of the corneal depends on its curvature and refractive index. Inappropriate refractive power leads to the loss of ability to focus images on the retina, called refractive error, or ametropia. Human eyes often exhibit three kinds of ametropia, including myopia, hyperopia and astigmatism, all of which are caused by an imperfection in the refractive function of the eye. Myopia, commonly known as near-sightedness, is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. The situation of hyperopia is the opposite to that of myopia. People who has astigmatism usually has a slightly irregular corneal, leading to more than one focus before or behind the retina. The method of surgical correction for ametropia, according to international standards, can be divided into corneal surgery, crystal surgery, and sclera surgery according to the different surgical sites.
The corneal refractive surgery generally includes LASIK (Laser-Assisted In-Situ Keratomileusis) and LASEK (Laser-Assisted Subepithelial Keratomileusis). In correcting myopia, the central stromal layer of the corneal is cut by laser. As a result, the overall corneal curvature is reduced, the refractive power is reduced, so the focus moves to the retina and the patient's vision is corrected. In the case of hyperopia, the peripheral stromal layer is cut to increase the overall curvature. The correction of astigmatism is a little complicated, because the curvature should be adjusted locally.
LASIK is the mainstream method to treat the refractive errors, but it is an invasive process, and often accompanied by some complications.